public async Task <IEnumerable <TextValue <int> > > Students(int workshopId)
        {
            var request = new AvailableParticipantsQuery
            {
                WorkshopId = workshopId
            };
            var data = await mediator.Send(request);

            return(data);
        }
        public async Task <IEnumerable <TextValue <int> > > Handle(AvailableParticipantsQuery request, CancellationToken cancellationToken)
        {
            var list = await ctx.Student
                       .Where(s => !s.WorkshopParticipant.Where(wp => wp.WorkshopId == request.WorkshopId).Any())
                       .OrderBy(a => a.Surname)
                       .ThenBy(a => a.Name)
                       .Select(a => new TextValue <int>
            {
                Value = a.Id,
                Text  = $"{a.Surname}, {a.Name} ({a.Email})"
            })
                       .ToListAsync(cancellationToken);

            return(list);
        }